A tractor-trailer brake system can basically comprise wheel brakes on the tractor and trailer as well as an engine brake on the tractor. A foot pedal operates the service brake which simultaneously applies all of the wheel brakes. For a long downhill run a continuous-duty brake, normally actuated by hand, may be set which operates the engine brake on the tractor and the wheel brakes on the trailer, the latter normally via a solenoid valve which can be provided on the tractor or on the trailer in the air line to the trailer brakes.
Such a brake system is required in some jurisdictions by law in all tractor-trailers over nine tons gross axle weight. Such a brake retards the downhill travel of a tractor-trailer and is operated normally on long slopes.
The main difficulty with such a split system is that the retardation force of the engine brake in the tractor and of the wheel brakes on the trailer do not maintain a fixed relationship to each other. The principal cause of this is that the engine typically operates with a uniform retardation force, whereas the retardation force of the tractor brakes varies.
More particularly in drum wheel brakes the retardation force decreases after long periods of brake operation. This is caused by thermal swelling of the brake drums which therefore move away from the brakeshoes and decrease the pressure inside the corresponding brakeshoe cylinders. The results is decreased retardation force, known as brake fade.
With disk brakes the opposite effect is noticed. More particularly after some operation the disk clamped between the brakehsoes swells and forces itself into more tight contact with these brakeshoes, thereby increasing the pressure in the cylinders behind them and correspondingly increasing the braking force.
Obviously when descending long hills it is essential that the braking force remain uniformly distributed between the tractor and the trailer. If the braking increases in the trailer considerable tension strain will be exerted on the coupling--fifth wheel or trailer hitch--between the tractor and the trailer. If the retardation force of the trailer brakes decreases the trailer will ride up on the tractor so as not only to overload its brakes, but to make a jackknife whereby the trailer runs partially past the tractor possible.
In drum-brake systems the above-described brake fade cannot be eliminated. The regulations in some jurisdictions allow a 40% fade when the continuous-duty brake has been operated continuously for 6 km. Thus it is normal procedure for the driver to periodically assist the continuous-duty brake at least at the end of a long downhill run by means of his or her service brake. This operation of the service brake, which is effective at least in the trailer on the same wheel brakes as are used for the continuous-duty brake, only aggravates any brake fade in the trailer brakes, greatly increasing the possibility of a jackknife. It is also not unknown for such brakes to become so very overheated in the trailer that the drum literally splits apart, resulting in serious accident.
In a split air-brake system using disk-type wheel brakes the driver must normally switch the continuous-duty brake off toward the end of a long downhill run as the braking force inside the rear disk brakes can become excessive. More frequently the driver switches the continuous-duty brake in and out during a long downhill run, giving the trailer disk brakes some time to cool off. This has the effect of greatly stressing the coupling between the tractor and the trailer, as the switching in and out of the rear trailer brakes whose kinetic retardation force may be greater or less than that of the engine brake in the tractor can occasionally lead to an accident wherein the tractor and trailer separate from each other. Furthermore when the continuous-duty brake is switched off to allow the trailer wheel brakes to cool the tractor engine brake is also switched off, thereby allowing the tractor-trailer to gain considerable momentum. In this situation the driver must operate the service brake so that in effect the trailer disk brakes are afforded little time to cool off.